Three reasons why power steering systems fail

You might be surprised to learn that one of the three principal reasons for power steering pump failures is forgetfulness.

To run a hydraulic power steering pump safely during a dyno run, simply form an oiling loop. Connect the power steering reservoir to the pump’s inlet port and the pump’s outlet port back to the reservoir’s return port. Then fill the reservoir with fluid, about 1in above the return port.

Ken Roper’s firm supplies hydraulic power steering pumps for the Aston Martin race cars and to all the Australian V8 Supercar teams

“With the best will in the world though,” says Ken Roper, head of KRC Power Steering, “it’s easier to remember to put oil in the engine than it is in the power steering reservoir, and the consequences of the oversight can be severe.” As most power steering pumps rotate at engine speeds, the internals of one without oil will soon turn blue and invariably weld themselves together. The same transgression is common when breaking-in a new camshaft or similar. Although break-in speeds are usually significantly lower, often around 2,500 to 3,000rpm, a dry power-steering pump may survive the torture, though it usually suffers some adverse consequences.

The second reason for power steering pump failure occurs when the oil hoses have not been thoroughly cleaned in the aftermath of a previous pump wreckage. Avoid this demoralizing calamity by ensuring all related hydraulic hoses are flushed clean. If not, the new replacement pump will be soon sabotaged by shrapnel from the previous failure.

Improperly routed hose

The third condition that causes power steering pump failure is improper bleeding or worse, disregarding the bleeding procedure completely. In common with a dry-sump lubrication system, expelling air bubbles from the power steering system is crucial. The worst situation is caused when air becomes trapped in an improperly routed hose, an up-loop, from which it cannot escape. As a consequence the inner diameter of the hose is drastically reduced, which starves the pump of fluid. Soon after the pump falls victim to a destructive condition called cavitation.

Cavitation inflicted two 1/8in deep cavities and blew the lugs off this rear cover! This drama was caused by installing an unauthorized inlet fitting.

Cavitation brings explosive pressure with devastating effects

In the pump’s rotor-vane arrangement, the vane draws the oil in, compresses it and it pushes it out through the outlet port. However, if air is present in the oil it will become compressed. “With velocity and heat,” says KRC’s Ken Roper, “the air bubbles function like a plasma cutter and will attempt to melt a hole at the area where most of the pressure is exerted.”

Air can be introduced by the aforementioned improperly routed hose or by reservoir fluid level that is too low. To address this plight we have assembled the seven principal steps to erase the notorious air bubbles that cause all the problems.

1) To begin the bleeding process and also to avoid imposing unnecessary pressure on the power steering system, raise the vehicle off the ground.

2) Fill the reservoir slowly and bring the fluid level near the top and leave the cap off. To fill the slave cylinder with fluid, turn the steering wheel from full left lock to full right lock several times. This action encourages fluid to circulate and expel the initial air bubbles from the system. Ensure the fluid level in the reservoir always remains about one inch above the return port in the reservoir.

3) Without starting the motor, either crank it over with the starter or remove the belt and turn the pump with a drill or, better still, fit another pulley in the drill, as shown, connect both pulleys with a belt and spin the pump pulley. Make certain the drill is rotating in the correct direction (clockwise when looking at the pump from the front) as the pump only performs in one direction of rotation.

4) While spinning the pump, enlist the help of an assistant to move the steering from side-to-side and begin the bleeding process. Because air gets trapped at the highest points, start the bleeding process at the highest point, which is usually the pressure side, the outlet port, of the pump. Then move systematically lower until the steering rack or steering box is reached. Follow the direction of flow and finish the bleeding process at the return fitting on the reservoir. Use a technique similar to that for bleeding brakes: loosen a line fitting momentarily then close it.

5) If the reservoir is located at the highest point of the system it will often self-bleed. However, vehicles fitted with a Hydro boost require attention first. The hydro boost is associated with some hydraulic brake setups and is operated by the power steering system rather than a conventional vacuum-powered brake booster. As such it will occupy the highest point of the system and thus should be bled at its highest fittings.

6) Continue to monitor the reservoir fluid level which, if maintained high above the return port, will allow the air bubbles to float out and up to the surface. Finally repeat the bleeding process. If the power steering pump squeals or makes grumbling noises or grinding sounds they indicate air remains in the system.

7) Some reservoir caps are vented others are not. KRC Power Steering is an advocate for the non-vented type which requires “burping”. When at normal operating temperature burp the pressure by loosening the reservoir cap at which time air will rush out. The fluid expands at normal operating temperature and displaces some of the air in the system, which builds pressure in the reservoir. When the cap is replaced and the engine stops the fluid cools and contracts, creating a vacuum in the reservoir. Under vacuum the fluid is free of aeration.

1 Comment

I have to say, this is one of the most informative reads on cavitation I’ve found, and I’ve read a lot of them… I had not considered hose routing as a possible problem in a pressurized system, and I think you may have just hit the head of the nail on my problem. And a very good explanation about the destructive effects of cavitation. Thanks very much.